1 . Field of the Invention
The present invention relates to a developing device for use in an electrophotographic apparatus.
2. Description of the Related Art
The structure of a conventional developing device will be described with reference to FIG. 1.
In FIGS. 1(a) to 1(c), reference numeral 1 designates a casing for accommodating a developing device. In the bottom portion of the casing 1, there is disposed a carrying magnetic roll 2 in such a manner that it can be rotated in a direction of an arrow a and, at the upper right portion of the carrying magnetic roll 2, there is disposed a developing magnetic roll 3 in such a manner that it can be rotated in a direction of an arrow b. Also, in the casing 1, there is stored a developer 4 in such a manner that the carrying magnetic roll 2 can be submerged in the developer.
In FIGS. 1(a) to 1(c), reference numeral 5 designates a doctor plate, 6 a photosensitive drum which forms an electrostatic image holder, 7 a developer peel-off plate, and 8 a developer supply device.
Now, description will be given below of the operation of the developing device having the above-mentioned structure when it performs a developing step.
If the carrying magnetic roll 2 and developing magnetic roll 3 start their rotational operations from a state shown in FIG. 1(a), then, as shown in FIG. 1(b), the developer 4 is drawn up by the carrying magnetic roll 2 and the developer 4, which has been attached to the carrying magnetic roll 2, is extended over to the side of the developing magnetic roll 3 due to a magnetic action between the two magnetic rolls 2 and 3.
The developer 4 absorbed to the developing magnetic roll 3 can be restricted to a given developer layer thickness by passing the developer 4 through the doctor plate 5. The developer, which has been scraped off by the doctor 5, is returned to the developer storage side, whereas the developer, which has been held by the developing magnetic roll 3 and has been carried to the photosensitive drum 6, is used to develop electrostatic latent images formed on the surface of the photosensitive drum 6 in a developing area D or the like. After passing through the developing area D, the developer held by the developing magnetic roll 3 is scraped away by the developer peel-off plate 7 disposed close to the developing magnetic roll 3. In the following description, in order to distinguish the developer before passing through the developing area D from the developer after passing through the developing area D, reference number 4 is given to the developer before passing through the developing area D and 40 is given to the developer after passing through the developing area D.
The developer 40 scraped off by the developer peel-off plate 7 is slid down on the upper surface of the developer peel-off plate 7 and then returned to the developer storage side.
When, in the conventional developing device having the above-mentioned structure, there is used a developer which is prepared by mixing a toner and a carrier at a given ratio, there are produced a portion (developer 40) in which the ratio of the toner to the carrier is reduced because the toner is consumed in the developing area D, and a portion (developer 4) in which the ratio of the toner to the carrier is not reduced because the developer is not carried into the developing area D and thus the toner is not consumed. Therefore, the amount of toner corresponding to the amount of consumption must be supplied to the developer consisting of the above-mentioned developer 40 every given period by the developer supply device 8.
However, in the conventional developing device, even although it includes the above-mentioned developer supply device 8, as the toner 9 is simply allowed to fall down to the developer storage side, the toner 9 and developer 40 cannot be mixed together well, with the result that, as shown in FIG. 1(c), there are produced in the casing 1 three layers, that is, a toner layer 9a formed of the toner 9, a layer formed of the developer 4, and a layer formed of the developer 40. Therefore, if the toner layer 9a is carried into the developing area D, a stained background phenomenon (a phenomenon that a toner stain occurs in the background of a printed matter) can be invited, or, if the layer formed of the developer 40 is carried into the developing area D, then a printed matter of a low density can be produced due to a shortage of the toner.